A~,lnitroso ccxnpounds c;isily react ;is nucleophilcs ~ith conjugated azoalkenes to give (~-(arylimino-N-oxide)hydraz~mes by their 1,4-addition to the a~x~-cnc system These adducts undergo an internal hetcr~x:yclizati~m process with pyra;/.ole ring formation to produce 1-alkoxycarbonyl-or I-amintx~lrtxmyl-3-methyl-4qarylimino-N~xidc)-IH-pyraz~-5(4H)-ones stereoselectively in Z form by loss of an alcohol molecule. Dcoxygenation of these complmnds with triphenylphosphine affords I-alkoxycarlx~nyl-or l-amin~x.'artxmyl-3-methyl-4-arylimino-IH-pyi-azx~l-5(4H)-ones. Basic treatment with triethylamine of the same comlxmnds leads to 3-rnethyl~l-(arylimino-N-oxidc)-lH-pyrazd-5(4H)ones by removal of the substltucnts on N( I ) hctcroatom of the pyrazolc ring. Both dcoxygenation and basic treatment of l-alkoxxcarbonyl-and 1 -amintx:artxmyl-3-methyl-4-(arylirnino-N-oxide)-lH-p)Tec/ol-5(4H)-ones have been realized sequentially, providing 3-methyl-4-arylimino-lH-pyr~x~l-5(4H)-tmcs. Thc same products were succcsfully obtai ned by rcvcrsing the order of thc~ processes.